1. Technical Field
The present disclosure relates to a radar device.
2. Description of the Related Art
In recent years, studies are being made of a radar device using radar transmission signals with short wavelengths including microwaves and millimeter waves by which high resolution can be obtained. Moreover, there is a demand for development of a radar device (wide-angle radar device) which detects objects (targets) including not only vehicles but also pedestrians in a wide angular range to improve safety outdoors.
For example, a pulse radar device which repeatedly transmits pulse waves is known as the radar device. A reception signal of a wide-angle pulse radar which detects vehicles and pedestrians in a wide angular range is a signal in which multiple reflected waves from a target (for example, a vehicle) existing in a close distance and a target (for example, a pedestrian) existing in a far distance are mixed. Accordingly, the radar device requires (1) a radar transmitter having a configuration which transmits pulse waves or pulse modulated waves having autocorrelation characteristics achieving low range side lobes (hereafter, referred to as low range side lobe characteristics) and (2) a radar receiver having a configuration with a wide reception dynamic range.
The following two configurations can be given as examples of the configuration of the wide-range radar device.
The first configuration is a configuration in which radar waves being pulse waves or modulated waves are transmitted by performing mechanical or electronic scanning using a directional beam with a narrow angle (with a beam width of about several degrees), and reflected waves are received by using the directional beam with the narrow-angle. In this configuration, scanning needs to be performed many times to obtain high resolution. Accordingly, a tracking performance for a target moving at high speed is poor.
The second configuration is a configuration in which reflected waves are received by an array antenna formed of multiple antennas (antenna elements) and which uses a method of estimating an arrival angle of each reflected wave by using a signal processing algorithm based on a reception phase difference corresponding to a space between the antennas (direction of arrival (DOA) estimation). In this configuration, estimation of the arrival angle in a reception branch can be performed even when some of scan intervals of transmission beams are omitted in a transmission branch. Accordingly, the scanning time is reduced and the tracking performance is improved compared to the first configuration. Examples of the direction-of-arrival estimation method include: Fourier conversion based on matrix operations; a Capon method and a linear prediction (LP) method based on inverted matrix operations; and multiple signal classification (MUSIC) and estimation of signal parameters via rotational invariance techniques (ESPRIT) based on unique value operations.
Moreover, there is proposed a configuration (hereafter, referred also to as MIMO radar) of the radar device which includes multiple antennas (array antennas) in the transmission branch in addition to the reception branch and performs the beam scanning by performing signal processing using transmission and reception array antennas (for example, see Jian Li, Petre Stoica, “MIMO Radar with Colocated Antennas,” Signal Processing Magazine, IEEE Vol. 24, Issue: 5, pp. 106-114, 2007).
In the MIMO radar, by arranging the antenna elements in the transmission and reception array antennas in a certain way, virtual reception array antennas (hereafter, referred to as virtual reception array) can be formed as many as the product of the number of the transmission antenna elements and the number of the reception antenna elements at maximum. This has an effect of increasing an effective aperture length of the array antennas with few elements.
Moreover, the MIMO radar can be applied not only to one-dimensional scanning in a vertical or horizontal direction but also to two-dimensional beam scanning in the vertical and horizontal directions.
However, when the number of antennas in the transmission and reception branch is restricted to achieve cost and size reduction of the MIMO radar (for example, when the number of transmission antennas is about four and the number of reception antennas is about four), the aperture lengths in the vertical and horizontal directions are restricted in the planar virtual reception array formed by the MIMO radar.